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Fix D12533: Simplify curve object to mesh conversion

This patch simplifies the curve object to mesh conversion
used by the object convert operator and exporters.

The existing code had a convoluted model of ownership, and did quite
a bit of unnecessary work. It also assumed that curve objects always
evaluated to a mesh, which is not the case anymore.

Now the code checks if the object it receives is evaluated. If so,
it can simply return a copy of the evaluated mesh (or convert the
evaluated curve wire edges to a mesh if there was no evaluated mesh).
If the object isn't evaluated, it uses a temporary copy of the object
with modifiers removed to create the mesh in the same way.

This follows up on the recent changes to curve evaluation,
namely that the result is always either a mesh or a wire curve.

Differential Revision: https://developer.blender.org/D12533